A typical vehicle safety seat belt system is designed to restrict the displacement of an occupant with respect to the occupant's seated position within the vehicle when the vehicle experiences a sudden, sharp deceleration. See commonly assigned U.S. Pat. No. 3,322,163. A typical seat belt system has three main portions: the retractor belt, the torso belt, and the lap belt and the performance of each belt may be characterized by its force-displacement curve. The area under the force-displacement curve is referred to as the energy absorbed by the safety restraint.
Current vehicle safety seat belts are made from fully drawn polyethylene terephthalate ("PET") fiber which is partially relaxed (2.7%) and having a tenacity of at least 7.5 g/denier and 14% elongation at break. However, a problem exists with current PET fiber seat belts. Crash studies indicate that after the initial vehicle impact occurs (e.g. speed of about 35 miles/hr), the occupant tends to move forward from his seated position until the belt engages to build restraining forces. As indicated in FIG. 1, the relatively unyielding belt made from PET fiber exerts a load of at least 2000 pounds (about 9000 Newtons) against the occupant so as to cause the occupant to have chest and rib cage injuries at the seat belt torso position and also neck and back injuries when the occupant rebounds and impacts the back structure of the seat assembly.
U.S. Government regulation requires that seat belts must withstand loads up to 6,000 lbs. When a car collides at a speed of 35 miles/hour, an impact energy to which an average sized person in the car is subjected is at least 500 Joules on the torso belt. Although the current PET fiber may absorb the impact energy, damage to the vehicle occupant still occurs due to the undesirable force-displacement curve. In 70 milliseconds, an average sized passenger will experience high forces of up to 2,000 pounds (about 9,000 Newtons) as shown in FIG. 1.
In order to absorb the impact energy and to reduce the seat belt load against the vehicle occupant, U.S. Pat. No. 3,550,957 discloses a shoulder harness having stitched doubled sections of the webbing arranged above the shoulder of the occupant so that the stitching permits the webbing to elongate from an initial length toward a final length at a controlled rate under the influence of a predetermined restraining force. However, the stitched sections do not give the desirable amount of energy absorption, do not provide uniform response, and are not reusable. See also U.S. Pat. No. 4,138,157.
U.S. Pat. No. 3,530,904 discloses a woven fabric which is constructed by weaving two kinds of yarns having relatively different physical properties and demonstrates energy absorption capability. U.S. Pat. Nos. 3,296,062; 3,464,459; 3,756,288; 3,823,748; 3,872,895; 3,926,227; 4,228,829; 5,376,440; and Japanese Patent 4-257336 further disclose webbings which are constructed of multiple kinds of warp yarns having different tenacity and elongations at break. The webbing shows multiple step gives and impact absorbent characteristics. Those skilled in this technical area have recognized the deficiencies in using at least two different yarn types as taught by the preceding references. U.S. Pat. No. 4,710,423 and Kokai Patent Publication 298209 published Dec. 1, 1989 ("Publication 298209") teach that when using at least two different yarn types, energy absorption occurs in a stepwise manner and thus, the web does not absorb the energy continuously and smoothly. Therefore, after one type of warps absorbs a portion of the impact energy, and before another type of warps absorbs another portion of the impact energy, the human body is exposed to an undesirable shock. In addition, these types of seat belts are not reusable.
U.S. Pat. No. 3,486,791 discloses energy absorbing devices such as a rolled up device which separates a slack section of the belt from the taut body restraining section by clamping means which yield under a predetermined restraining force to gradually feed out the slack section so that the taut section elongates permitting the restrained body to move at a controlled velocity. The reference also describes a device which anchors the belt to the vehicle by an anchor member attached to the belt and embedded in a solid plastic energy absorber. These kinds of mechanical devices are expensive, are not reusable, provide poor energy absorption, and are difficult to control. An improvement on the foregoing devices is taught by commonly assigned U.S. Pat. No. 5,547,143 which describes a load absorbing retractor comprising: a rotating spool or reel, seat belt webbing secured to the reel; and at least one movable bushing, responsive to loads generated during a collision situation, for deforming a portion of the reel and in so doing dissipating a determined amount of the energy.
U.S. Pat. No. 4,710,423 and Publication 298209 disclose webbing comprised of PET yarns having tenacity of at least 4 grams/denier and an ultimate elongation of from 50% to 80%. Due to the inherent physical properties of PET yarn, the Examples show that, at 5% elongation, the load has already reached more than 700 kg (about 1500 lbs). The damage to the occupant by seat belt still exists and thus, the belt needs to be further modified. Examples in these two patents also show that if PET yarn is overrelaxed, the tenacity drops to 2.3 g/denier.
Kokai Patent Publication 90717 published Apr. 4, 1995 discloses high strength polybutylene terephathalate homopolymer ("PBT") fiber based energy absorption webbing. The fiber's tenacity is over 5.8 g/denier, breaking elongation is over 18.0%, and the stress at 10% elongation is less than 3.0 g/d. However, this reference fails to teach PBT fiber demonstrating the initial stress requirement which engages the seat belt to protect the occupant and the means to control the initial stress barrier.
It would be desirable to have an improved energy absorbing seat belt which has a smoother performance than that of the known stitched webbing approach or the known use of at least two different fibers, is reusable unlike the known clamp approach, and also addresses the ability to control the initial barrier stress and the impact energy absorption.